The helicopter was carrying out a security mission between the city of Kirkuk and Salahuddin province in the early morning when it came under heavy fire and crashed in western Baiji, 180 km (112 miles) north of Baghdad, police said.

It is the second helicopter to be downed in the area in recent weeks.

It was not immediately clear who was behind the attack, but the area is a stronghold of al-Qaeda's Iraqi wing, which merged with its Syrian counterpart earlier this year to form the Islamic State of Iraq and the Levant.

The Sunni Islamist group has claimed responsibility for attacks on both sides of the border, striking almost daily in Iraq in a campaign to destabilize the Shi'ite-led government and trigger wider sectarian confrontation.

In a separate incident, three people were killed when a roadside bomb blew up at the entrance to a small market in Nahrawan, 30 km southeast of Baghdad, police said.

The United Nations Mission in Iraq said nearly 900 civilians were killed across Iraq in September, raising the death toll so far this year to well above the total for 2013.